Lighting control system with packet hopping communication

ABSTRACT

Building lights are master controlled to reduce power consumption under building master control, or in response to electric utility commands to the building computer. Each lighting wall control unit includes a transceiver which can communicate to at least one neighbor transceiver, thereby forming a distributed communication network extending back to the building computer. The transceivers operate asynchronously with low data rate FSK signals, using carrier frequencies between 900 and 950 MHz. Different communications protocols control packet forwarding and acknowledgement so that messages reach their destination but are not forwarded in endless circles, and so that collisions are minimized.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to concurrently filed application, Ser. No.08/498,286 by Srinagesh Satyanarayana for PACKET HOPPING SYSTEM WITHSLIDING FREQUENCY, AND TRANSCEIVER FOR THE SYSTEM, and concurrentlyfiled application, Ser. No. 08/498,285 by Srinagesh Satyanarayana forTRANSMITTER CONTROL SYSTEM FOR LOW DATA RATE FSK MODULATION.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to centralized control of building systems havingmany devices or elements distributed throughout the building. Forexample, the invention is applicable to centralized control ofartificial lighting systems in buildings where each room or area has anindividual local control, or to heating and/or air conditioning systemshaving many individually controllable heating devices, heat pumps(heating or cooling) or heat exchangers. The invention is alsoapplicable to occupancy-sensing, security, or fire detection systemsrequiring sensed-data transmission from a multiplicity of sensorlocations to a central location, as well as control signals transmittedback to selected locations, and systems for improving energyconservation by providing a combination of individual and centralcontrol of lighting levels or temperatures at many or all locationswithin a building, or permitting temporary reduction of energyconsumption on a most effective basis in the event the supply system orutility is overloaded or limited in capacity.

More particularly, in lighting applications this invention relates tomethods and apparatus for communication between a building computer anddozens or hundreds of modules which each control one or more lamps orluminaries within a room or area.

For convenience, this specification and the claims refer extensively toa "building" or "building computer." It should be clear that the term"building" should be interpreted as including a portion of a building,or a building complex having two or more structures or portions thereofunder common control, and sharing one network; and might be applicableto an amusement park or other outdoor situation.

Other applications not directed at energy conservation might includecentralized override of volume or channel settings for an existinghard-wired background music and public address system, and in particularwould allow control on a group or zone basis where this had not beenprovided in the hard-wiring layout.

2. Description of the Prior Art

The need to reduce electric power consumption during emergencies, or asa result of simple excessive demand on an electric power system, has ledto increasingly sophisticated solutions and proposals. In the past, whenan electric utility needed to reduce consumption, it could only reducedistribution voltages or request (in some cases demand) that usersreduce their consumption. However, voltage reduction is undesirable, inpart because it reduces the efficiency of many of the devices consumingpower, and in part because many of the energy consuming devices respondnon-linearly to voltage reduction.

Reduction of power consumption involved two kinds of problems:communication, and actual reduction. The first depended largely onannouncements over public radio channels, and telephone calls tooperators of large businesses and buildings. The second problem washandled with mixed success. Raising temperature settings or turning offselected air conditioning units reduced this aspect of system powerconsumption without blocking use of buildings. However, becauseindividual light switches were either "on" or "off," and lightingcircuits were not arranged to permit effective reduction of lightingwhile maintaining safe operations in the buildings, significantreductions in this major aspect of energy use often impaired work outputor building usability. Within a building, compliance often requiredsending maintenance personnel to each switchboard or office to turn off"unnecessary" lighting circuits.

With the development of centralized building computer systems forcontrol of heating, ventilating and air conditioning (HVAC), directcommunication between electric power distribution system computers andbuilding computers became a possibility. However, this did little toimprove the effectiveness of power consumption for artificial lighting.

At the same time, innovations in the control of individual fluorescentluminaires, and groups of luminaires on the same circuit, have openedthe door to dimming artificial lighting levels efficiently withoutmaking task performance impossible in the affected areas of rooms orbuildings. The Varitron® system is an example of fluorescent lightingcontrol systems which allow highly responsive control in an area largeenough to require a number of luminaires, but sufficiently uniform inuse or light need so that one control regime provides satisfactoryillumination for the entire area. This system provides a remotelycontrollable ballast in each luminaire, and controls all the ballasts inthis area by low frequency amplitude modified signals transmitted overthe AC power line from one wall unit. Dimming of luminaire output to anumber of discrete levels, such as 110%, 75%, 50%, 25% and 9% of normal,provides sufficient versatility while at the same time simplifying thecontrol signalling and responding functions. Within the area localcontrol may be provided by a user operable infrared based "dimmingmouse" and/or an occupancy sensor; and a programmable wall unit can beused to change settings automatically at preselected times of day ordays of the week.

Central "on-off" building control of lighting has been possible for manyyears, through use of low voltage wiring from a central office orcomputer to operate relays controlling local power circuits. However,this technique does not lend itself well to dimming control, because ofthe large numbers of conductors which are required to carry signals fordifferent dimmer control relays, and the special adaptation of theindividual room lighting controls which is required. Such a system isvery difficult to retrofit in an existing building. A further difficultywith this type of system is that correction of wiring failures is oftendifficult because wiring drawings are inaccurate or non-existent, andtracing these low voltage cables is time consuming and expensive.

A low-power radio control system for receivers up to 75 feet away isdescribed in a paper "SONA/ECS, a decentralized environmental controlsystem (for disabled)" published as part of the Proceeding of the IEEEComputer Society Workshop on Computing to Aid the Handicapped, Nov. 4-5,1982.

Another system for building control makes use of the existing AC powerwiring in the building to carry control signals. So-called "carriercurrent" systems for impressing a relatively low radio frequency on ACpower wiring have been used for telemetering data in power distributionsystems, and for "wireless" intercom or music systems, but have beenprone to excessive noise. To reduce cost, 900 MHz radio channels for2-way communication are described in "900 MHz radio provides two-waypath for control and return" is described in Transm. and Distribution,vol. 36, no. 6, pp 33-6 for June 1984. This system had the advantagethat it was claimed to be installable and maintainable by the utility'sown workforce.

Systems using high frequency spread spectrum techniques for distributingcontrol signals are described in U.S. Pat. Nos. 5,090,024, 5,263,046,5,278,862 and 5,359,625. The last of these approximates a sweptfrequency waveform in which successive square waves are formed by thechirps of the sequence, and the frequency of the square waves is variedover at least a portion of the sequence. This technique is suggested foruse in the 900+ MHz band.

A control system sold under the name Echelon uses microprocessors forcontrol of direct link communication to each of the individual controlsof the network over a common channel. This system caters to a widevariety of applications, and can have as much as a 1 Mbit/seccommunication capacity. This system uses a communication protocol whichspecifies a packet structure, handshake commands to set up acommunication and acknowledge a communication, certain error correctionand recovery, and retransmission after a time delay if a communicationis lost. Transmission is possible over various media, such as twistedpair, radiated RF, infrared, or high frequency signals carried on thepower line, between the central source and each of the nodes, exceptwhere a relay may be provided to a group of nodes. As a result,installation of such a system is expensive and requires considerabledevelopment time. The Echelon system can be used in the license-free 49MHz band when power is less than 1 watt. Especially if an RF signal istransmitted over the power lines, this system employs a spread spectrumencoding to provide noise immunity.

As communications systems have become more sophisticated, variousprotocols have been developed to improve communication efficiency whenit is not practical to have a direct link between source anddestination. For example, a routing code can be transmitted with themessage, and the entire route is directed by this code. Alternatively,in systems such as the telephone networks, complex algorithms evaluatethe available paths to select the one which is optimum at this time.However, such systems require substantial computing power at eachnetwork node, and are not practical for an in-building distributedcommunications system.

SUMMARY OF THE INVENTION

An object of the invention is to provide a central building controlsystem which can supplement or override local control, and whichrequires little or no addition to a building's fixed wiring.

Another object of the invention is to provide a building control systemwhich has a distributed communication system using low cost buildingblocks.

Yet another object of the invention is to provide such a system whichcan easily be installed by electricians without special training.

A further object of the invention is to provide a building lightingcontrol system which is easily installed by retrofitting into anexisting building.

Still a further object of the invention is to provide a centralizedcontrol system with redundant route capability compensating forunreliability of individual links.

According to the invention a building control system includes individualcontrol units in each of the rooms or areas to be controlled; a centralcontrol unit for generating control signals which are directed to aparticular one, or a group, or all of the individual control units; anda low power radio transceiver associated with each of the individualcontrol units and with the central control unit, where each of thetransceivers transmits sufficient power to communicate with at least oneother of the transceivers, but not all of them; and at least some of thetransceivers can exercise at least some control over settings of theassociated individual control units, overriding at least the priorsettings established by local operation of the individual control units.Different levels of priority can be assigned to individual controlpoints, to determine to what extent either the central or local controlcan override the other. It is clear that the small number of bits neededto transmit dimming information leaves room in a packet, as short as 5bytes, for extensive priority coding.

According to a preferred embodiment of the invention each transmissionby the transceiver associated with the central control unit, which isintended for a particular individual control unit, is a packet ofdigital information including an address of the transceiver associatedwith that control unit, and the control signal. Optionally the packetmay also include the sender's address, routing or re-transmissioninformation, priority coding, and/or various check bits. Instead of aparticular address, the packet may contain a group address, or be codedas an all-network broadcast. The digital information packet may bepreceded by a burst of unmodulated carrier signal, and/or asynchronizing signal. These will be selected according to thepredictability of the carrier frequency, and the type of modulation,being used, so that receivers can lock on to a transmission before thefirst information (e.g., address) bit is received.

Each combination of a control unit and transceiver includes circuitryfor determining, from address information in a packet received by thattransceiver, whether the packet is intended for that transceiver; and ifnot, whether or not the packet should be retransmitted. If the receivedpacket is addressed solely to that transceiver, in accordance with anycontrol data contained in the packet, the combination will provide anappropriate control signal to the devices in that room or area.

Where a received packet is addressed to at least one other combination,re-transmission decisions are based on routing or re-transmittingdecisions; that is, if a route is defined, and this transceiver is anintermediate node along the route, this transceiver will retransmit thepacket, whereas if routing is not defined, the decision whether or notto retransmit will be based on some other criterion. In a systemaccording to the invention the route can, for example, be fully definedby the address in the packet, or by the address and routing data in thepacket, or by a routing table which is stored in the combination (inthis case, probably two tables, one for outgoing packets and one foracknowledgements). If transmission is by "flooding" then examples ofretransmission criteria include comparison of the packet with recentlytransmitted packets, and retransmission codes.

To eliminate need for pre-determining routing paths and to reduce thepossibility of collisions, the preferred mode of packet hopping isflooding with hop counts. According to a further aspect of the preferredembodiment, each packet which is transmitted includes a code signalindicative of a maximum number of times that packet should beretransmitted; and prior to retransmitting the packet, a transceiverdecrements that code signal. In a still further preferred embodiment ofthis aspect of the invention, a transceiver/control unit contains amemory for storing a previously retransmitted packet, and a circuit forcomparing the latest received packet with that stored packet todetermine if a retransmission criterion is met.

According to another aspect of the preferred embodiment, when atransceiver/control unit combination has received a packet for which itis the "destination"(address of combination is the address in thepacket) upon providing the appropriate control signal to its controlunit, the transceiver will transmit an acknowledgement signal. Ifflooding is used in both directions, any transceiver receiving theacknowledgement signal will attempt to retransmit the acknowledgementsignal. If routing information is transmitted and kept with a packet,then this would be used in the reverse direction for acknowledgement. Ifa re-transmission limit is used for outgoing packets, then a similarscheme may be used for acknowledgement packets, or acknowledgements maybe treated differently.

Where the system being controlled is a building's artificial lighting,the control units may provide local on-off and/or dimming control, andcentral control of dimming or dimming and on-off, or other combinations.Different priority levels can permit local users greater or lessercontrol overriding the central control signal. For example, top localpriority can permit resetting to any light level by local control;second priority allows two steps of brightness increase but not above100%; third priority allows one step of increase but not above 100%; andlowest priority allows no increase under local control. Preferably thepriority is provided as a code with the central control signal and willvary according to the room or area being controlled, and thecircumstance causing the control signal to be transmitted.

Preferably, most or all the transceivers are interchangeable andtransmit with approximately the same power level in a frequency bandwhich provides good penetration of structural walls and floors, butwithout radiating substantially into other buildings or suffering severeinterference. This frequency can be any available commercialtransmission band which has suitable propagation characteristics.However, it is also advantageous to select a band, such as an "ISM"band, which permits unlicensed operation if the power output is lessthan a certain figure, such as one watt. A desirable band meeting thosequalifications, and for which relatively low cost RF equipment isreadily available, is the 900 to 950 MHz band; but other bands such asISM bands near 49 MHz, 470MHz, and 2.4 and 4.5 GHZ may be considered. Inone embodiment, the transceivers are closely regulated for transmittingfrequency, to fall with one channel; and the receiver section monitorsonly that channel. Upon receiving a packet which it should retransmit,the transceiver will wait for a period of time, and will then retransmitin that same channel unless the transceiver detects presence of acarrier signal within that channel. This period of time is preferablyobtained from a random number table stored or generated in thetransceiver/control unit combination; but the delay may be a selectedvalue pre-assigned to that combination.

In another embodiment, to reduce cost the transmitter frequency is nothighly stabilized, especially with respect to temperature. Transmissionfrequency will fall somewhere within a relatively broad band, ratherthan in a defined channel. In this embodiment the receiver section iscapable of detecting transmission at any frequency within thatrelatively broad band, and locking on to that frequency to detect thedigital signal. Upon receiving a packet which it should retransmit, eachtransceiver will wait for a period of time, and will then retransmit atwhatever in-band frequency its transmitter is then ready to produce,unless the transceiver detects presence of a carrier signal within thefrequency band being used for communication. Again, this period of timeis preferably obtained from a random number table stored or generated inthe transceiver/control unit combination.

Preferably, all the transceivers are interchangeable and transmit withapproximately the same power level, using a carrier frequency which isbetween 900 and 950 MHz; and still more preferably, within a nominalband approximately 10 MHz wide, such as approximately 905 to 915 MHz. Iftransmission is controlled to fall within one channel, this ispreferably a channel of approximately 30 kHz bandwidth.

Further, according to the invention a method of controlling at least oneparameter at a plurality of control points distributed throughout atleast a portion of a building includes the following steps:

transmitting a first radio signal, having a first power level sufficientto be received reliably at least one of the control points, andinsufficient to be received reliably at all control points within thebuilding, from a transceiver associated with a central control point,

modulating the first radio signal with a packet of digital informationincluding address information relating to at least a second controlpoint, and a control signal relating to the controllable parameter forall control points defined by that address information,

receiving the first radio signal at the one of the control points,

transmitting a second radio signal, having a second power levelsufficient to be received reliably at another of the control points, andinsufficient to be received reliably at all control points within thebuilding, from that one of the control points,

modulating the second radio signal with a packet of digital informationincluding address information relating to at least the second controlpoint, and the control signal,

continuing reception and retransmission of packets containing addressinformation and the control signal until the second control point isreached, and

upon receipt of the packet at the second control point, controlling theparameter according to the control signal.

In a preferred embodiment, when a control signal relates to all thecontrol points in the building, the address information includes an "allnetwork" address or code, and the method further includes controllingthe at least one parameter at the one of the control points, accordingto the control signal, retransmitting a packet containing that controlsignal from successive control points until all control points havereceived one of these packets, and controlling the at least oneparameter at each of the control points.

In still another preferred embodiment the first and second radio signalsare transmitted in a same frequency band, and the method furtherincludes:

after receiving the first radio signal, testing at said one of thelighting control point, to determine if a further radio signal at somecarrier frequency is being received in that same frequency band, and

transmitting said second radio signal only after determination that noradio signal at approximately the same carrier frequency is beingreceived. According to another aspect of this embodiment, before testingto determine if the band is clear, the method includes waiting either apredetermined period of time assigned to that transceiver, or a periodof time randomly determined by that transceiver.

A further feature of the invention, especially valuable at the time ofinitial installation, provides a method of automatically determiningusable communication links between the different control points. Thismethod uses unique identification codes which are (preferably) installedin each transceiver by the manufacturer, and which are recorded oninstallation sheets or drawings when the transceiver/control unitcombinations are physically installed; and acknowledgement signals whichare transmitted from a control point whenever it receives a packet whichis addressed to it or a group of which it is a member, the wholenetwork. To achieve automatic network evaluation, the building computertransmits signals successively to each of the nodes, to cause them totransmit test packets. From the patterns of acknowledgement signalsreceived the computer can generate displays or printouts showing theuseable links and the locations of the nodes in the building.

The ability to automatically determine usable links, and from this todetermine the number of hops which should be required to reach anyparticular node, provides the additional benefit that a change in thephysical location of the building computer, or apparently minor buildingrenovations which affect one or more links, can be accommodated easily.The initial installation method can be repeated, so as to re-define thesystem, with very little cost in operator time.

One of many applications of the above-described method embodiments isfor controlling lighting levels within a building having a plurality oflighting control points. In this application, the control points arelighting control points from which lights in an area or room can bedimmed or turned on and off; and the parameter is a lighting level(e.g., off, dimmed to some level, or normal on). This method furtherincludes controlling at least a first luminaire from the one of thelighting control points which receives the first radio signal,independent of that first radio signal.

In a preferred embodiment, when a lighting control signal relates to allthe control points in the building, the method further includescontrolling at least a first luminaire from the one lighting controlpoint, in response to receipt of the first radio signal, and

controlling at least one other respective luminaire from each of theother lighting control points, in response to receipt of one of theradio signals.

It will be clear to those or ordinary skill in the art that all thesefeatures provide low cost wireless communication, with a very low costof installation.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic drawing of a system according to the invention,showing communication links which are expected to be functional betweendifferent nodes,

FIG. 2 is a diagrammatic view of a building in which the system of FIG.1 is used,

FIG. 3 is a diagram showing the relationship between different levels ofcontrol of building lighting,

FIG. 4 is a node logic diagram for the system of FIG. 1 communicatingover partially pre-planned routing,

FIG. 5 is a node logic diagram for the system of FIG. 1 communicating bypseudo-random flooding,

FIGS. 6a-6d are diagrams of packets usable with different operationalprotocols, the packet of FIG. 6c being adapted for the system of FIG. 5,

FIG. 7 is a block diagram of the lighting control system in a room ofthe building of FIG. 2,

FIG. 8 is a time diagram showing transmitter frequency sliding,

FIG. 9 is diagram showing receiver lock-on,

FIGS. 10a and 10b are block diagrams of the transmitter and receiverportions of a transceiver according to the invention, with simplesliding frequency, and

FIGS. 11a and 11b are block diagrams of the transmitter and receiverportions of a second transceiver according to the invention, having abreakable transmitter phase locked loop.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The system shown in FIG. 1 demonstrates the principle of the inventionas it might be applied to a building shown diagrammatically in FIG. 2,having rooms 11-14, 21-25, and 31, 32, 34 and 35. A number of RFtransceivers T11-T14, T21-T26, T31, T32, T34, T35 and T41 form acommunications network. All but two of these transceivers haverespective associated room lighting controls C11-C14, etc. which controlthe built-in fluorescent lighting in the respective rooms, and whichreceive control signals from the transceiver. The transceiver T26functions as a radio relay and therefore does not have an associatedlighting control. The system includes a building computer 40 directlyconnected to the transceiver 41 which can communicate with thetransceiver/control combination T11/C11 for the lights in that room, andpreferably at least one or two other transceivers of the network. Itwill be clear, of course, that the building computer and its associatedtransceiver do not need to be in the same room (they are usuallyconnected by a cable), and the transceiver T41 can be located anywhereso long as it can reach and be reached by at least one othertransceiver.

Almost any small computer will have sufficient processing power andstorage capacity for use with the invention. An "application" programwill provide procedures for network set-up, normal operation (bothautomatic and as instructed by building personnel), routine networktesting, and any desired interface with other computers or sources ofcontrol.

The different levels of control made possible by the buildingcommunication system are shown in FIG. 3. The highest level usesapplications programs in computers; these may also be considered asintelligent software modules which reside primarily in a buildingcomputer, but may also automatically interact with another computer in,for example, an electric utility serving the building. The second levelof control is the building manager, who will have at least some power tomodify or override the normal control modes of the building computer.The third level is the communications network itself, because partialfailure of, or changes in, this network affect the ability of the higherlevels to control room units. The lowest level is user control, whichcan range from an indefeasible on-off switch (ultimate authority) to alimited permitted variation in the dimming setting of some or all of theluminaires in a room.

To permit use of standard "building blocks" for assembling systems, andavoid the administrative complications of facility licensing, it ispreferable to select a transmitter power and carrier frequency for whichunlicensed operation is permitted, but which can transmit through atleast one interior wall or floor of a building so as to provide reliablecommunication with at least one other transceiver. It is also desirableto minimize interference (false reception of lighting control signals)by a similar system in adjoining building. In a preferred embodiment ofthe invention, each of these transceivers operates in the same frequencyband, such as the UHF band between 900 and 950 MHz, and preferably theband between approximately 905 and 915 MHz, where pulsed transmissionbelow 1 watt power is permitted. In a building of typical commercialoffice construction, power levels between 30 and 100 mw, for example,appear desirable. By limiting the duration of each transmission burstto, for example, 100 msec, and observing a waiting interval slightlyover 6 seconds, the average transmitted power can be maintained belowthe equivalent of 460 μw continuous. The paths of reliable communicationbetween transceivers T11-T41 are shown by the interconnection lines inFIG. 1.

To minimize the effect of failure of any one transceiver, the networklayout in the building has been designed to provide at least twonormally reliable communication links for each room transceiver/controlcombination. Typical intra-building problems whose resolution is shownby this embodiment include the lobby 14 on the ground floor, which mayhave special ceiling decorations or features interfering withcommunication with the room 24 directly above, and the end room 23 whichdoes not have reliable communication directly with combination T22/C22because of the length and utilization of room 22. This problem isovercome by providing transceiver T26 partway along the length of room22 to relay messages.

In the system shown, signals normally originate from the buildingcomputer 40. This computer will frequently itself be connected via amodem or other network to a power company (electric utility) computer,to provide automatic control in the event of emergency conditionsrequiring reduction of power consumption in a region. The transceiverT41 transmits signals which are coded either to control a designated oneor group of the wall units C11-C35, or all the units.

Another aspect of the invention relates to the control of retransmissionby the various transceivers, so that messages will eventually reachtheir destination, and an effective compromise can be reached betweenoverall system control complexity and confusion due to multipletransmissions of the same message.

Stored Routing Tables

The operational scheme shown in FIG. 5 uses a predetermined routingarrangement, with each node operating asynchronously, and retransmittingon a CSMA (Carrier Sense Multiple Access) basis. Each transceivercontains an address table of those nodes (or groups of nodes) for whichmessages are to be routed through this node. When a message is receivedat step 110, it is error checked in step 112, and in step 114 thedestination address in the packet header is compared to determine if itis directed to this node. If so, in step 116 the control message isdecoded and performed, and in step 118 any directed immediate change inlighting is checked.

In step 120 the destination address is again checked, to determine ifother nodes should receive the same message. If not, then anacknowledgement control signal is output. If this is a group address,then in step 122 the address table is checked to determine if themessage should be retransmitted. If YES, then in step 124 the channel ischecked for signals indicating that another transceiver is transmitting,and in step 126 the message is retransmitted as soon as the channel isclear. If the step 120 determination was that the address was anindividual address, or in step 122 the group address was determined notto be in the address table (that is, this node is not in the pre-setpath to any more remote nodes), or the step 126 transmission has beencompleted, then in step 128 an acknowledgement signal is generated.

If, contrary to the sequence described above, in step 114 the receivedpacket address is determined not to be this node, then in step 130 theaddress table is checked to determine if this packet address is listed.If this answer is YES, or step 128 has been completed for a messageintended for this node, then steps 134-136 are performed similarly tosteps 124-126, thereby transmitting either the message intended for theother node, or the acknowledgment of receipt by this node. After thestep 136 transmission, or determination in step 130 that the message isnot intended for a more remote node on this pre-set path, then in step138 the receiver is re-initialized to await any other messages.

The process of FIG. 4 presumes not only that each transceiver ortransceiver/control combination has (typically, has been programmedwith) a unique node address, but also that address tables for groupaddresses and node addresses have been provided and loaded. The pre-setroutings represented by these address tables can be pre-determined froma study of the building layout, but are subject to required modificationin the event of failure of a transceiver near the beginning of a path,or degraded or blocked transmission between two transceivers which areadjacent on the path. Thus, in addition to power-up initialization instep 140, and network initialization in step 142, discussed below,automatic re-configuration of the pre-set routing may be required ifrepeated failure of the building computer to receive acknowledgementsignals from one or more transceivers indicates either a hardwarefailure or a communication link blockage.

As described above, the FIG. 4 configuration and operating method mayincrease both costs of hardware and network overhead for initialprogramming and re-programming within the building computer and at theaffected transceivers. In a building having a number of communicationlinks whose reliability changes frequently because of building usedynamics this re-programming could be a serious disadvantage. This canbe overcome by providing two or more routing paths to particularcombinations, for example by including additional addresses in thetables searched in step 130.

Flooding

An alternative communication method involves "flooding" the building,such that each transceiver repeats received messages withoutconsideration of logical or pre-set routing, unless a received messageis addressed solely to that node. However, it is clear that there mustbe some limitation, or else a message would be endlessly circulatedaround the network. One form of limitation is to put a date/time cut-offcode in the header when it is transmitted from the building computer,and to inhibit re-transmission by any node after that time, and to put asimilar cut-off code in the header of acknowledgement messages. However,this method requires that each transceiver/control combination contain alocal clock whose time is accurate, at least in comparison with the timefor a packet to be retransmitted to its final destination.

Another method of limiting message circulation around the networkinvolves inserting a sequence number in the header, and providing eachtransceiver with a memory stack which has a pre-set number of locationsfor storing the most recently received sequence numbers, andre-transmits a message only if its sequence number is not found in thestack. Again, however, this method requires additional memory capacityin the transceiver/control combination.

Flooding with Hop Counts

A method of limiting message circulation, without requiring large localmemory capacity, utilizes a kill level variable code placed in theheader of each transmitted packet. This technique takes advantage of thefact that the building computer contains data defining the number ofrelay steps normally required to reach each node in the building, sothat it is easy to limit the number of re-transmissions to zero fornodes in direct communication with the building computer's transceiver;to one, for "second tier" nodes, and so on; and to allow one or twoextra retransmissions for far-removed nodes where collisions on the mostdirect route may cause a message to arrive by a slightly longer route.This method also prevents a given node from retransmitting a secondtime, when it receives a retransmission of a packet from another nodeafter the given node has retransmitted the packet.

The logic diagram of FIG. 5 shows operation of a node when thetransmission protocol includes a packet kill level variable. Steps110-120 are the same as in FIG. 4. After determination in step 120 thatthe address was a group address, however, in step 150 the packet isstored in a packet memory. This packet header includes a fieldcontaining a kill level variable, which is a number that indicates howmany times this message may be re-transmitted. In step 151 this field ischecked, and if the value is greater than zero, in step 152 the killlevel variable is decremented once. In step 154 this modified packet isheld while the channel is checked, as in step 124 of FIG. 4.

If the channel is clear, then in step 126 the packet with decrementedkill level variable is transmitted. After a predetermined period of timein step 154, if the channel has not become clear the attempt toretransmit is aborted. Following aborting in step 126, or identificationof an individual address in step 120, an acknowledgement packet isgenerated in step 128. Even though aborting means that nodes fartherdown the group will not receive the packet at this time, if this nodewaits too long to transmit the acknowledgement signal, then the buildingcomputer will repeat the packet to this node as well as well as others.

In step 160 the acknowledgement packet is held while the channel ischecked, as in step 134, and in step 162 the acknowledgement signal istransmitted.

If in step 114 a received packet is determined to be addressed to adifferent node, then in step 166 it is compared to the previouslyreceived packet which is most recently stored in the packet memory. Thismemory may be sized to store only the most recently received packet or,where traffic is high in a large network, the last two or more may bestored for comparison. If the address and data content are the same,then in step 168 the packet kill level variable in the memory iscompared with that of the packet just received. If the variable in thejust-received packet is the same as or less than the stored value of thevariable, and is greater than zero (that is, the just-received packethas arrived here by the same number as, or fewer retransmissions than,the one previously received, so a new repeat packet will be forwarded),or if the comparison in step 166 showed that this message is a differentmessage, then in step 170 this packet is stored in the same memory asused in step 150. In steps 172, 174 and 176 the packet kill levelvariable is decremented, the resulting packet is put in the one-stepqueue for transmission if the channel becomes available soon enough, andis transmitted as in steps 152, 154 and 156. After transmission ofacknowledgement in step 162 or the other node's packet in step 176, orloss of transmission criteria in step 168 or 174, in step 138 thereceiver is re-initialized to await receipt of the next packet.

The node processing of FIG. 5 prevents retransmission of a recentlyretransmitted packet upon receipt of that same packet as a result ofretransmission by a node further removed from the building computer,while at the same time accepting for retransmission a repeat messagefrom the building computer, resulting from failure to receive anacknowledgement signal after a previous attempt to send this packet toanother node.

Acknowledgement packets may be handled just like outgoing packets,except that they always will be directed to the building computer.Therefore, instead of providing that address, an acknowledgement codemay be used. The combination (which was the destination for the packetoriginally) can then leave its address in the same location in thepacket, and set the kill level variable to a predetermined value storedin that combination's memory.

Partitioned Spanning Tree (PAST)

For a building which is not large, and in which the necessarycommunication links are reliable, an efficient (in terms ofcommunication resources) routing protocol uses partitioned addresses,which combine a unique address and routing information in one field inthe packet. Although the packet length may be increased, for a networkwhere the longest chain is 10 or fewer links, this packet length isacceptable.

The packet format shown in FIG. 6d uses two bit positions to representone four-level digit of an address. If messages are limited toall-network, and individual node address, then theoretically 255 roomscan be addressed with a 4-digit address using four-level digits, but inpractice the realizable number will be much less.

Preferably, at each node few, simple comparisons should be required todetermine if a message is to be retransmitted. The PAST format shownuses one digit to identify the first level nodes (e.g., 1000); the nextdigit to identify nodes reached through that first level node (1200 isthe second node reachable through node 1000); and so on. An address of0000 is recognized as an "all-network" message and is retransmitted byall but the last-level nodes.

According to this protocol, at first level node 1000 any address havingthe first digit =1, and at least one other digit unequal to 0, should beretransmitted. Second level node 1200 will retransmit addresses startingwith 12, and having either later digit unequal to zero; and so on. Thisrouting scheme greatly simplifies the logic and memory requirements atthe various nodes; but 2-bit digits limits the tree to 3 nodes at eachlevel of branching, except for the last level which can accommodate 4nodes for each of the 9 third level nodes. Thus, if the farthest roomscan be reached in 4 hops, and the starting point is near the center, atotal of 147 rooms could theoretically be addressed uniquely. However,it is unlikely that signal propagation in a building will be thatfavorable, so that many of the addresses would be unusable.

In the system just described, an address of 0000 is a network-widebroadcast. However, no use is made of addresses 0001 through 0333. Amore efficient operation can use a leading bit to distinguish between anetwork-wide broadcast and an acknowledgement signal. With thisaddressing scheme, more of the following bits are available foraddresses; if "1" means acknowledgement, then the following bits will beused for the address of the node. This arrangement enables a nodereceiving such an acknowledgement packet to retransmit only if it comesfrom a node farther out on the same branch of the tree, and therebyinhibit multiple retransmissions of the same acknowledgement signal bydifferent nodes at the same level.

One problem which is not readily overcome is that of collisions betweenacknowledgement signals following a network-wide broadcast. Inparticular, all nodes have identical transceivers and microprocessorcapabilities. None of these include storage of packets in a queue forretransmission. Therefore, if two nodes transmit, in rapid butnon-colliding succession to the same higher level (closer to thebuilding computer) node in the tree, it is likely that the higher levelnode will not yet have retransmitted the first of these acknowledgementpackets, and one will become lost. This cannot be resolved easily bysetting staggered delays in transmission of acknowledgement of anetwork-wide broadcast, where these delays are simply related to thenode address, because certain nodes will receive strong signals fromnodes which are in other branches, and are capable of preventingaccurate detection of signals from nodes in the same branch.

Increasing the length of the address, by increasing the number of levelsor increasing the number of bits per digit, allows use of this schemefor buildings with a large number of rooms. However, in the event thatone of the communication links becomes unreliable, reconfiguration ofthe routing can be very difficult.

Signal and Data Format

For reasons to be discussed below, the carrier frequency is very highcompared to the relatively small amounts of data required for buildingcontrol. A very low data rate such as 4800 baud will suffice. Oneproposed format and transmission plan involves a transmission cycle ofapproximately 200 msec; that is, the building computer will wait thatlong to receive an acknowledgement signal. Failure to receiveacknowledgement within that time period is considered proof of failure,so that the message will be resent.

Alternatively, when the farthest nodes will require many hops to bereached, the building computer may base the time before re-sending of amessage on the number of hops required for the round trip, plusallowance for some waiting time before each of the re-transmissions.

A packet may consist of 20 8-bit bytes transmitted at a 20 Kbit/sec bitrate, preceded by an unmodulated carrier burst or preamble lastingperhaps 12 msec. This corresponds to a total transmission duration of 20msec. At least the first two bytes will be allocated to address and/orother data, including routing information, which will identify a singleroom control, or a group, or all controls, to which the message isdirected. Only 3 bits are required for lighting brightness (dimming)information. Additional bits will be allocated for check bits, andacknowledgement or other system command information.

The different packet formats shown in FIGS. 6a-6d are not drawn toaccurate time scale. The length of one block is not necessarily onebyte, or an integral fraction of or number of bytes. Blocks which, for agiven size building and control arrangements, may be identical, have thesame reference numeral.

Direct Routing

Perhaps the least sophisticated technique, because it increases packetlength the most, is to transmit a complete routing path as part of eachoutgoing packet. This has the advantage of simplicity in processing ateach node; however, in a medium large building having unique addressesfor each node, eight to twelve bytes may easily be required.

As shown in FIG. 6a, a packet 50 starts with a header or preamble 51,which may be unmodulated or contain synchronization or other bitmodulation to simplify identification of the packet as valid for thisnetwork. The header or preamble will have a length determined by therelative difficulty (amount of time required) for a receiver to lock onto the transmission, for decoding and acting on it. The firstinformation block 52 is the address of the node or transceiver whichoriginated the packet; for outgoing packets this would be the address ofthe transceiver T41 connected to the building computer. The next blockis the route block 53, which contains information describing the routeto be followed between the source and the destination. The route block53 may be as short as one address, as shown, when the destination is asecond-tier transceiver; many bytes in length if the destination can bereached only after many retransmissions; and will be omitted if thedestination is a first-tier transceiver; or may be coded in some fashionto reduce packet length in a large building.

In the format of FIG. 6a the fourth block is the destination block 55,which is the address of the combination for which the packet's controldata is intended. The address can be completely arbitrary, or cancontain portions identifying the building (useful if adjacent buildinginterference is a recognized problem) as well as addresses withingroups. This is followed by a command block 56, which may containvarious kinds of network information or packet description, such as"acknowledgement," or the packet length, or priority information; or maydesignate that some special response is required of the combination,such as transmitting a test signal. The data block 57 may be very short.In a lighting control network, settings of "off," or dimming to 9%, 5%,50%, 90%, normal, or 110% may be used, as an example. This can be codedin only three bits.

The check block 58 is the last transmitted in most formats. This mayfollow any desired error checking or correction routine, and may be moreor less than one byte in length.

When the FIG. 6a format is used, the entire route information may bepreserved when the packet is retransmitted, or the address of the nodedoing the retransmitting may be deleted from the list. When the fullroute is retained to the destination, then the generation of theacknowledgement packet is simplified, because all of its routeinformation already exists. However, this protocol requires that areceiving combination must check the entire route and destination blocksto determine if the packet should be retransmitted, decoded forcontrolling this control unit, or ignored. If any of the addresses,except the destination, does match that of the receiving combination,then the packet should be retransmitted. When writing the applicationprogram, those of ordinary skill will be able to devise other protocolsbased on use of this packet format, to best fit local needs.

Direct Addressing

Because it reduces packet length and the amount of retransmission, thenode logic of FIG. 4 may appear most elegant. After the header orpreamble, the ordinary data packet 60 shown in FIG. 6b need only containthe address 55 of the destination node; flags or other control codescontained in the command block 56; the lighting control data in block 57which may require as little as 3 bits; and error check or correctingblock 58. However, it is preferred that the source block 52 is retainedin this embodiment. This block can, for example, be used to distinguishbetween outgoing and acknowledgement packets, and therefore can reducethe number of different logic operations required in the combinations.

Different packets may be of different lengths, usually because ofdiffering lengths of the data field 57. When using the packet of FIG. 6bwith the node logic of FIG. 4, to initialize the network the ROM's ofeach transceiver's microprocessor must be loaded with address tables.Other examples of extraordinary data include error correctionalgorithms, which may reside in fixed ROM when the control unit ismanufactured, or may be loaded later. Thus initialization data may bemuch longer than routine lighting control data. The packet length may beone of the items coded in the command block 56.

Flooding with Hop Counts

Where the system designer wishes to avoid the requirements imposed bythe FIG. 4 logic and FIG. 6b packet on memory and logic capacityrequired in each combination, due to storage of address tables and thenecessity of multiple comparisons during data reception, and the networkburden of up-dating these tables whenever substantial changes in signaltransmission between nodes is discovered, then a packet format such asthe packet 70 of FIG. 6c may be preferable. Blocks 52-58 may beidentical to those of FIG. 6b. The particular feature of the FIG. 6cformat is block 79, which includes the "kill level"variable. Typicallythis block is set when a packet is transmitted by the buildingcomputer's transceiver T41. It has a value related to the greatestnumber of hops that transmission along a normal, or slightly longerroute, will take. This field value is decremented each time the packetis retransmitted, and is not retransmitted when the received packet hasa kill level of, for example, 0. This prevents packets from beingcirculated endlessly around the network, without any need for complexaddress or routing schemes.

PaST Format

The Partitioned Spanning Tree format 80 shown in FIG. 6d providespredetermined routing with only simple storage and comparison functionsin each node. The sole difference from the packet of FIG. 6b is theaddress formatting for the source block 82 and the destination block 85.These addresses are arranged in a tree structure, starting from thetransceiver T41. The tree arrangement is based on the movement of apacket along successive links, which are the branches of the tree,outward from the transceiver T41. All first level nodes must be indirect communication with the transceiver T41.

The address is shown with respect to the source address 82. Each addressis formed by a series of digits each occupying a sub-field 82' withinthe address field 82. In this embodiment a digit is represented by twobits 84, so that its numerical value can range from 0 to 3. Packets areretransmitted by a node if all but the last digit match this node'saddress, and this node is not a last level node (which neverretransmits).

It will be clear that in all of the above packet descriptions, thevalues given are merely exemplary. The relative placement of the blockswithin the packet has been selected for convenience in processing, anddoes not form part of the invention. The functions in the command block56 and the check block 58 can be increased or varied, and neither ofthese blocks must consist of contiguous bits.

Collision Avoidance

To reduce the overhead burden of complex header and routinginstructions, in a medium size office building operation according tothe FIG. 5 method is preferable. The building computer will initiatetransmission of a packet such as the packet 70 shown in FIG. 6c, andwill then wait a predetermined period of time to receive anacknowledgement.

As shown in FIG. 1, transceivers T11, T14 and T24 can be expected toreceive this packet simultaneously. As will be described in greaterdetail below, each transceiver/control combination includes at least onemicroprocessor (hereinafter referred to as "the microprocessor" eventhough its total functioning may be divided between two processors),which can decode the received digital information and determine whataction, if any, is to be taken. If the message is to all controls, thenan appropriate control signal is provided to controls C11, C14 and C24.At the same time, each of the transceivers T11, T14 and T24 will prepareto retransmit the message. The first step in retransmission is toobserve a random delay interval, intended to reduce likelihood ofcollisions. Using a random number generator, the microprocessor of eachof these three transceivers produces a delay interval number, forexample between 1 and 128 periods. The duration of one delay period isarbitrarily selected, based on the transmitter power-up delay plus thedetection response time for the transceivers of this system, but willusually be less than one packet period or a small number of packetperiods. During the respective delay intervals, each of the threetransceivers will listen to determine if another network transmission isbeing received, and in the absence of detecting such transmission willcommence transmitting after its own randomly generated delay interval.

By coincidence for the system arrangement of FIG. 1, no othertransceiver can reliably communicate with more than one of the innermosttier formed by T11, T14 and T24. Therefore after T11's delay interval,T21 will receive the message; after T14's delay, T12 will receive themessage; and after T24's delay, T22, T25 and T34 should receive themessage. Assuming that there are no collisions, then this process willbe continued as the message is relayed, after various delay intervals,radially outward. However, there is significant risk that T22 will bethe victim of collision between transmissions from T24 (innermost tier)and T12 (next tier outward) because as drawn neither T24 nor T12 canreliably hear the other. If either T12 or T24 begins transmitting beforethe other has completed the message, the microprocessor of T22 willdetermine, through error coding, that corrupted data have been received.T22 will continue to wait for a clean message, and may receive sucheither from T25 or from T26. Because transmission from T26 would be afourth tier retransmission, while that from T25 is second tier, likelyof collision between them is greatly reduced.

Because the locations of the room lighting controls C11-C35 are oftendetermined primarily for reasons other than RF communication with othertransceivers, such as convenient access by people entering or within theroom, or historic accident of building wiring, network topography havingsome of the collision problems, due to differing numbers of links alongsomewhat parallel paths, will be common. One solution is to providepreferred routing, by increasing the delay for some nodes or providingother logical restrictions that reduce the likelihood of collision. Thecontrol programs resident in the microprocessors of the individualtransceiver/control combinations can be made more or less responsive toreprogramming by the building computer, to overcome parallel pathproblems. For example, transceiver T12 could be commanded not to relayany received messages; T13 would then communicate via T26 and T22. Uponcommunication via that route becoming unreliable, T26 could be disabledtemporarily, and T12 enabled for relaying through command from thebuilding computer.

Installation

A feature of the invention is the low cost of installation and set-up.Because a transceiver (including any associated microprocessor) isself-contained and requires no electronic adjustments, it is easilyinstalled by an electrician without special training. The onlyconnections required are input power, and control connections to thecontrol unit. Where justified economically, either the control unit orthe transceiver can be a plug-in unit to the other, or they can beintegrated. The only extra requirement is that a serial number, bar codenumber, or other number related to an address or identifying numberstored in the transceiver (usually by the manufacturer) be recorded oninstallation sheets or building drawings for each location.

A preferred embodiment involves a set-up routine which forms part of theapplication program. It can be fully performed automatically withouthuman intervention unless the result found is that one or moretransceivers, on the list of those which were installed, cannot becontacted.

In this method of network set-up, the building computer 40 initiatestransmissions of packets directed to those control points (first tiernodes) which can communicate directly with the transceiver associatedwith the building computer; and then transmits similar packets directedone at a time through each of the first tier nodes to those controlpoints which can communicate with the first tier nodes (therefore calledsecond tier nodes); and so on until at least one communication path hasbeen identified to each of the control points. From this information thebuilding computer calculates routing or retransmission data for each ofthe control points.

Using the node logic of FIG. 5 and the packet format of FIG. 6c, tostart the computer will command transmission of an "all network" addresspacket having a kill level of 0. This packet will be received bytransceivers T11, T14 and T24, these being the only ones withincommunications range. Each of these will not retransmit the packetbecause the kill level variable is 0, and will transmit anacknowledgement. Upon receiving these acknowledgements, the applicationsprogram will identify these three nodes as first tier, for creation of anetwork communications diagram like that of FIG. 1.

As a next round, one at a time the computer will direct a packet to thefirst tier nodes just identified, containing special command or datablocks which cause the addressed node to transmit an "all-network"packet with a kill level variable of zero. Upon receivingacknowledgement signals, these will be retransmitted to transceiver T41,so that the computer can identify the linkages from that first tiernode. Similarly, this method can systematically contact everytransceiver that is operably linked, so that the communications linkagediagram of FIG. 1 is created by the computer.

If, as an example, there is at this time no relay transceiver T26, andthe link from T13 to T23 is too unreliable, then an error message willbe generated under control of the application program, that there is nocommunication with T23. From study of the building diagram of FIG. 2(which can also be created in the computer, based on installationdrawing data which have been entered) building personnel can determinethat the problem is due to the isolation of transceiver T23, not atransceiver failure. The addition of relay T26 then becomes an obviouscorrection.

These data can be used in different ways, depending on the mode ofrouting selected for use in this network. For example, if normal modetransmission is by flooding, with transmission of a code signalindicative of the number of times a packet is to be retransmitted, thebuilding computer then stores a number associated with each controlpoint address, where that number equals the number of retransmissionswhich were required to reach the control point via the shortest route;or a greater number if the system operator determines that this willreduce the instances in which packets do not get through following thefirst transmission from the building computer's transceiver. This methodis especially economical if each control point has a unique serialnumber or bar code (as many as 48 or 50 bits may be used), which can bewritten onto a building map when the units are installed throughout thebuilding; and which can be used as an initial address when the controlpoint acknowledges receipt of a packet. Subsequently it will often bedesirable for the building computer to assign addresses which can be farshorter, for use in routine addressing of packets.

These techniques make it possible to display a 3-D image of the buildingon a screen, so that for future control purposes a human operator canselect control points for lighting optimization according to theirlocation with respect to outside factors such as sun exposure, or groupswhich are aligned along a wall or floor, without resort to complex listsand drawings.

If route information is normally to be transmitted along with theaddress in each packet (FIG. 6a), then the building computer stores theshortest route determined for each control point, for transmission ifthat control point is to be addressed. If address tables forretransmission are stored in each control point, then the computercreates such tables from the route information which was obtained, andsubsequently transmits the respective table contents to each controlpoint.

It will also be clear that, with this invention, other automatic set-uptechniques are possible under application program control.

Alternatively, a portable computer can be carried through the buildingto communicate one at a time directly with each of the control points,and determine which other control points this point can send a packetdirectly to and receive an acknowledgement signal from.

Transceiver/control unit combination

It is desirable that the control unit be either a standard unit, or onewhich is simply modified for easier connection to the associatedtransceiver. The arrangement shown in FIG. 7 is a preferred arrangementfor controlling lights with a room or area having common lighting needs.The room is shown as having 3 luminaires 202 each having one or morefluorescent tubes 203 and a remotely controllable ballast 204. In thisembodiment the ballasts are controlled via signals which propagate overthe supply conductors from a wall unit 210. The wall unit 210 includesan I/O (input/output) circuit 212 which provides the AC power to theluminaires, and also provides in-room communication and controlfunctions. For example, this room includes a "mouse" 214 whichcommunicates with the I/O circuit by an infra-red link and an occupancysensor 216 which also communicates with the I/O circuit by anotherinfra-red link. The mouse 214 and the sensor 216 provide control signalswhich are forwarded to a microprocessor 218 which stores lightingcontrol data or signals, and controls the I/O circuit so that the lightsare operated at the times and brightnesses desired (e.g., mouse 214control) or permitted by the building computer.

Connected to the microprocessor 218 is another microprocessor 222 whichis directly associated with the transceiver formed by receiver section224 and transmitter section 226.

This embodiment shows separate microprocessors 218 for room control and222 for network communication, and separate antennae 228 and 230 for thereceiver and transmitter. However, there is no reason that economy ortechnical developments may not dictate combining the microprocessors,and the antennae, into one each.

As will be discussed below, the receiver and transmitter sectionsoperate independently of each other, except that the communicationprotocol preferred for the invention requires that transmission andreception be mutually exclusive. In the preferred embodiment, thefrequency of transmission over antenna 230 is independent of the carrierfrequency most recently received over antenna 228.

Sliding Frequency

Interference from other signal sources is always a potential problemwith radio communications. People in unrelated industries areconsidering use of bands between 900 and 950 MHz because of commercialdevelopment of transceivers for operation in these bands, because of thepropagation characteristics of these bands, and because of thepossibility of unlicensed operation at low power. As a result it isnecessary to devise equipment or techniques which, while inexpensive,will provide sufficiently reliable data transmission for the buildingsystem being controlled in the face of unexpected interference.

Applicants believe that, in most cases, interference will come fromsignal sources which are also narrow band sources. By usingfrequency-shift-keying modulation, with a frequency shift of ±4 kHztransmission and a 20 Kbit/sec data rate, transmission according to theinvention is also a narrow band source. If successive packettransmissions from the transceiver T41 are at different frequencieswithin the band, then it is unlikely that all will be victims ofexcessive interference. One technique is to have a preplanned frequencyhopping scheme, such as is used for military security, but this requiresnot only storage of the hopping algorithm or sequence, but also tightsynchronization of the network transceivers to a master clock.

According to another aspect of the invention successive transmissionsare at carrier frequencies which are shifted by a frequency greater thanthe bandwidth of the transmitted signal, with a simple smooth variationof the carrier frequency or a stepwise monotonic variation across theselected bandwidth. FIG. 8 shows a preferred triangular waveformvariation in which, between one transmission and a next approximately1.5 sec later, the carrier frequency has shifted 1.5 MHz. This isreadily performed by the transmitter 226, shown in slightly greaterdetail in FIG. 10a than in FIG. 7, in which a VCO 302 receives a sweepinput from a frequency sweep control, and binary data input preferablyat 20 Kbits/sec. The rate of sweep is selected to keep the carrierfrequency change, during one transmission burst, within the receiverbandwidth, but also to maximize the chance that an interference signalwhich excessively degrades reception of this burst will not affect thenext transmission.

The VCO therefor will have a slowly varying frequency when transmittinga carrier burst at the beginning of a packet, and will alternate up anddown 4 kHz from the present carrier frequency when being modulated. Thissignal is amplified in power amplifier 304 and fed through antennacoupler 306 to antenna 230.

This causes the shift during one packet transmission (20 msec durationincluding a preamble burst) to be less than 20 kHz, well within the 50kHz preferred bandwidth of the receivers 224 of nodes withincommunication range, and their AFC circuitry.

If, to reduce costs or to allow a simple sliding frequency, a phaselocked loop is not used in the transceiver sections then temperaturevariations will cause variations of typically 10 PPM/° C. in the VCOfrequency. At 900 MHz this amounts only to 900 kHz variation for 100° C.variation between very cold building and cold unit, and hot buildingwith high ambient within the control unit. If aging has an equal effect,temperature and aging effects in the transmitter section will cause upto approximately ±1 MHz variation. To stay within a 10 to 12 MHz passband of receiver sections, the transmitter range chosen is variationfrom 905 to 915 MHz. The additional variation due to temperature andaging will, however, be essentially constant during the triangle periodof 20 seconds, so that the slope of frequency change will not exceedapproximately 1.0 Mhz per sec.

In order to simplify network control problems, it is desirable that thenetwork operate asynchronously, both as to timing of packet transmissionbut also as to oscillator frequencies. Thus, not only will thetransceiver T41 associated with the building computer have afree-running triangular wave control of its transmission frequency, witha period of approximately 20 sec for one full wave cycle, but so alsowill all other transceivers in the network. Each transmitter will beturned on only when transmission is commanded, and requests fortransmission can be made at arbitrary times, so that the exact frequencybeing transmitted will be unpredictable. The result of this protocol isthat the receiver section 224 of each transceiver has no way to predictthe frequency of the next data signal to be received, and must be ableto search and lock on to a transmission before the first data bit ofthat transmission.

To permit operation in this mode, each receiver has two operating modes:a capture mode, and a tracking mode. In the capture mode the receiverhas a pass band of approximately 10 to 12 MHz, from approximately 904 to916 MHz. As previously described, a packet commences with an unmodulatedcarrier burst lasting a sufficient period of time to allow the carrierto be detected, and for the receiver to lock on to that signal and trackit in the tracking mode. This is a function of the receiver scanningrate, and the detection and evaluation time. To keep circuitry costs andprocessing time down, an unmodulated burst of at least 1 msec isdesirable, and preferably approximately 12 msec.

In the tracking mode the receiver should have a narrow bandwidth, but nonarrower than 100 kHz. This effectively blocks interfering noise orsignals outside the narrow pass band, but passes the FSK signal fully.The receiver will incorporate an AFC circuit which is operable tocontrol the local oscillator in the tracking mode, so that the smallvariation in carrier frequency, if linear sliding is used, can befollowed.

A first embodiment of the receiver section 224 according to theinvention, shown by the schematic block diagram in FIG. 10b, includesthree sections: one broad band for the capture mode defined by band-passfilter 312, and one narrow band for the tracking mode, defined by IFfilter 322, and low pass filter 332. The signal from the antenna 228 ismatched to the amplifier 314 by a coupler 316. The output of a VCO 317is mixed with the output of filter 312 in mixer 318, to provide a firstIF signal. This is then mixed with the output of local oscillator 324 inmixer 325 to provide a second IF signal which is amplified in amplifier326 and filtered in second IF low-pass filter 332. A simple detector 336provides the detected binary signal from the FSK signal output fromfilter 332.

To identify the existence of an in-band signal while in the capturemode, a received signal strength indicator 340 also receives the outputof filter 332. The output of the received signal strength indicator 340is provided to a decision circuit 342. The decision circuit has oneoutput which controls the VCO 317, to cause the VCO to stop at thefrequency which provides a maximum output from indicator 340. In thisembodiment the bandwidth of the lowpass filter 332 is also varied by abandwidth controller 344, which receives a second output from thedecision circuit 342. The filter 332 may be set for a relatively widebandwidth during capture, and a narrow bandwidth during tracking so thatthe signal to noise ratio can be improved. The decision circuit 342 alsoreceives the output of the data detector 336 so that it can determine ifthe signal being received is a network signal, or is an interferingsignal. As soon as it is determined that a received signal isinterference, sweeping of the VCO is resumed to search for networksignals.

To improve the receiver's ability to discriminate against interferencesignals at a nearby carrier frequency, it is preferable to operate thereceiver with a relatively narrow bandwidth at all times. In thatcircumstance the bandwidth controller 344 is omitted.

In an alternative embodiment a tunable narrow bandpass filter is sweptacross the band such as 912 to 924 MHZ, to a point at which the signalis received. At that point sweeping is stopped, and the signal isdetected.

Avoiding Interfering Signals

Although at the time this invention has been made, there is relativelylittle traffic in the 900 MHz ISM band, systems according to thisinvention must be capable of operating without significant modificationor maintenance for many years. As a result it should be probable that,despite interference by other signals, and without need formodification, adjustment or reprogramming, the transceivers willactually receive network signals which have been transmitted.Interference can be caused by signals present in one part of thebuilding, but not others; and the interference can be specific signalswhich are frequently or always present on one channel, or signalsoccurring sporadically or randomly.

When a transceiver locks on to a signal, and assuming that this occursduring the unmodulated or preamble portion of a transmission, the signalcan be identified as a network signal either by the preamble, if any, orby the subsequent modulation and digital information being according tothe communication protocol used by the network; for example, themodulation type and bit rate matching one of the examples described inthis application, or another one selected for the system. Thisvalidation of the signal will probably require between 2 and 5 msec. If,however, the receiver and its microprocessor can more quickly detect thepresence of a different modulation, then this received signal can bemore quickly identified as spurious, and the transceiver will resumesearching for a different signal sooner. By including an additionaldetector which can detect the presence of "wrong" modulation, skippingcan commence in less than one millisecond. This slight increase in costand complexity of the receiver reduces the chance of missing thebeginning of a valid network transmission.

A further technique for interference avoidance, which may be preferredwhen a plurality of interfering signals are being detected within theband of interest, is prediction. If the receiver oscillators are nothighly stable and accurately calibrated, the carrier frequency of aninterfering signal cannot be identified accurately so that, as aspecific frequency, it is skipped during the next scan of the band.However, it is not necessary that the frequency itself be known. Iftunable filters or VCO's in the receiver are swept across the band by asource which is stably repetitive, the time from beginning of the sweepto the frequency corresponding to the interfering signal is easilymeasured and stored. An "interference frequency" table in themicroprocessor then stores a plurality of times which have beenidentified, on recent sweeps, as corresponding to interfering signals.For the next given number of sweeps the detector output is blanked whilethe oscillator or filter is passing these frequencies, so that onlynetwork signals or new interference signals are detected.

To further improve probability of detection of network signals by a"smart" receiver, in the presence of a number of interfering signals,the technique just described can be enhanced by checking each of thestored times, after a certain number of sweeps, which number may berelated to the number of entries in the interference frequency table.Each time that interference is verified as still present at a frequency,the time interval until this frequency is again checked is increased, upto some maximum.

The receiver frequency correlation described above can also be used toallow transmission on one frequency while already receiving another, ifdesired. According to the preferred operating protocol, a transceiverwill not transmit while it is receiving a network signal in the band. Asa practical matter, while the transmitter section is transmitting, thereceiver section may receive an overpowering signal such that only thecenter frequency of the strong signal can be determined. However, if thereceiver section has sufficient selectivity to receive and demodulate anetwork signal while the transmitter section is transmitting, then avariation in the control protocol may increase total packet throughputmore than it increases collisions. To accomplish this, the transceiver'scontrolling microprocessor must determine that this transmitter wouldnow transmit on a frequency sufficiently different from that beingreceived, to avoid collision at any other transceiver which is alsoreceiving the same packet as this transceiver. Otherwise it ispreferable to delay transmitting from this transceiver while a packet isbeing received. Such a determination can made without need foraccurately calibrated receiver and transmitter sections, while stilladhering to a preferred mode in which the transmitter frequency isvaried, if an approximate relationship can be determined between (a) thefrequency with which this transmitter section would transmit at thistime and (b) the frequency that the receiver section is now receiving.

With a transceiver as shown in FIG. 10, this relationship is determinedby causing the receiver to track the transmitter frequency while it istransmitting, as described above. For example, the microprocessorcorrelates the times in the receiver frequency sweep while so tracking,with the times or voltages in the transmitter's triangular or othervarying control circuit which are then causing this frequency oftransmission, and stores these correlations. When a network signal hasbeen detected and is being received, the microprocessor compares thetransmitter control value now being generated, with the value whichcorresponds to the receiver sweep time for this reception, anddetermines if the approximate frequency that would now be transmitted iswell separated from the network frequency now being received.

PLL Transmitters

The systems described above need not be operated with a linearly slidingfrequency, if interference can be otherwise avoided. For example, alltransmitters can operate at one frequency, and each receiver can betuned for that one frequency. This eliminates the possibility of missinga transmission because a receiver's scanning is interrupted while itevaluates signals which turn out to be interference; and it enables thelength of a preamble or synchronizing period to be greatly reducedbefore transmission of the first data bit begins. Intra-systeminterference due to collisions at individual receivers can be minimizedby the use of variable delays between receipt of a packet andre-transmission.

It is also possible to optimize system operation for use with aplurality of predetermined frequencies, so that the overall bandwidth orsweeping range of the receivers can be reduced; or each receiver can bearranged or programmed to scan only all or a selected group of thosepredetermined frequencies. To operate at a predetermined frequency, orscan on certain predetermined frequencies, typical practice is to use aphase locked loop to stabilize the frequency of a VCO against areference source, using selected division ratios for the selectedfrequencies.

The loop settling time is one of the important parameters to bedetermined when designing a PLL. If different frequencies are to begenerated at different times, then rapid settling is usually desired sothat the transmitted frequency is substantially constant at the newvalue shortly after the change in frequency has been commanded. Intelecommunication systems using PLL transmitters with FSK modulation,the data modulation rate is usually so high that the time period of thelongest allowable series of same value bits is small compared with theresponse time of the PLL. Therefore the modulation does not affect thecenter frequency which is being defined by the PLL. However, the systemdescribed in this application uses a low bit rate, and may transmitsignals having a series of same value bits which is longer than thesettling time. In this situation, the transmitter phase locked loopcauses the loop control voltage applied to the VCO to change--that is,the carrier frequency drifts from the selected value. This drift causescorruption of the data detected by the receiver, because in the receiverits first oscillator is locked at a frequency which differs from theselected value by exactly the first IF frequency.

In a packet data transmission system according to this aspect of theinvention, the transmitter phase locked loop is broken (opened) justbefore sending the data; that is, after the preamble, and just before orat the instant that modulation begins. Although the transmitter is"drifting" during the data modulation period, this time is short enoughthat actual drift should be inconsequential. As soon as the data bursthas been transmitted, and the transmitter's output amplifier has beenturned off, the loop is closed again, so that the oscillator is againstabilized at the selected (or at the next selected) frequency.

The transmitter shown in FIG. 11a includes components which may beidentical to those described with respect to Fig. 10a. The referenceportion of the PLL includes a reference oscillator 352 whose output isreceived by a controllable reference divider 354 whose output in turn isone of two inputs to a phase comparator 356. The output of the VCO 302is received by a controllable main divider 358 whose output in turn isthe other input to the phase comparator 356. The output of thecomparator 356 is passed through a switch 360 to a loop filter 362 whichmay have a settling time of approximately 2 to 3 msec. The loop filteris designed so that, in the absence of a signal input to the filter, itsoutput will remain substantially constant for a time period equal to thelongest data burst to be transmitted. The output of the loop filter 362is one of two inputs to a summer 364 in the VCO, which also receives thebinary data signal to be transmitted. The VCO output is amplified inpower amplifier 304, and provided to antenna 230.

The reference divider 354, divider 358 and switch 360 are controlled bysignals from the transceiver microprocessor, such as the processor 222shown in FIG. 7. Changing the divider ratios allows selection ofdifferent predetermined frequencies. The switch 360 is preferably openedby the microprocessor just before the first data bit to be transmitted,and closed immediatly after completion of the data packet. It will beclear that opening of the switch 360 can be delayed slightly, so long asthe change in the control voltage from the loop filter 362 produces anoscillator change which is small compared with the frequency deviationused in the FSK. transmission.

The receiver portion shown in FIG. 11b has a similar PLL control of theVCO 317, through controllable reference divider 374 and main divider378, which are also controlled by the microprocessor 222, and acomparator 356. The loop filter 362 may be identical to that used in thetransmitter portion. Optionally, it may also be economical to use oneset of PLL circuits both for transmission and reception, although theVCO frequency will be set differently to provide the IF frequencyoffset.

If each transmitter transmits only at an assigned frequency, theadjustable dividers in the transmitter PLL can be simpler, fixeddividers; and if the whole system uses one frequency, the receiverdividers likewise need not be adjustable. However, there still may beeconomy is using one set of PLL circuits for both transmission andreception, in which case the dividers must be adjustable to permitshifting frequency by the amount of the first intermediate frequency.

Other Variations

Many other uses and variations of the invention will suggest themselvesto those of ordinary skill. For example, by making the packets larger,they can be used for transmitting much larger bursts of information,such as audio.

The invention is not limited to low bit rate modulation, nor to FSK.These are desirable for a particular lighting control application, butother modulation techniques or rates may be preferred choices for otherapplications, especially any requiring a higher data transfer rate.

It is not necessary that acknowledgement packets be processed in thesame way as outgoing packets. For example, regardless of the format foroutgoing packets, it may be desirable to minimize multiple transmissionof them. To accomplish this, according to this variation anacknowledgement packet should contain a code identifying it as anacknowledgement, as well as the address of the acknowledging node. Eachnode is programmed to store the address of the next node along the routeto the building computer. Only the acknowledgement code and that "nextnode" address need be added, when transmitting an acknowledgement ofreceipt for this node. When received at the "next node" the address willbe identified as valid. Because this is an acknowledgement, this "nextnode" will substitute its stored address for sending acknowledgements,and retransmit. This continues until the building computer is reached.

This technique suffers the disadvantage that some reprogramming isrequired if one of the links becomes sufficiently unreliable, eventhough parallel links were available.

Especially after an "all-network" packet has been sent, it is desirablethat there not be such a large number of collisions that manyacknowledgement signals are lost. Assuming a transmission duration of 10msec, and a total of 400 control units in a building, it would require 4seconds for the building computer's transceiver to receive all theacknowledgements if they arrived in a perfectly concatenated string.Thus it may be desirable that, after an "all network" packet isreceived, each transceiver apply a longer than usual random delay beforeattempting sending an acknowledgement signal or retransmitting onereceived from another node. Similarly, the building computer shoulddelay far longer than usual before sending any individually addressedpackets to combinations from which an acknowledgement packet has notbeen received.

If the "building" actually consists of two structures which are spacedsufficiently far apart that direct radio communication from at least onenode in one to at least one node in the other is unreliable, then asingle building computer can control both by providing a data line fromthe computer to a transceiver in the remote building. The problem ofinterfering packets can probably be minimized, however, by consideringthe two structures as one network. It may even be most economical tolink them by placing a relay transceiver on the exterior of one of thebuildings, or both, similar to the way that the relay T26 is used in theembodiment of FIGS. 1 and 2.

Many other formats or protocols can be used to avoid effects ofinterference. There is no requirement for frequency sweeping if one ormore channels are available for substantially exclusive use of thisnetwork; in that case, all transceivers can operate on a same channel ifthe transmitter stabilities are adequate. However, this will create thedisadvantage of increased collisions. With frequency sweeping by eachtransceiver, independent of the others, the possibility of collision ata receiver is greatly reduced. Any particular receiver section which haslocked on to a first transmission will not usually be affected ifanother node, within reception distance, commences transmitting on afrequency which is outside the narrow pass band to which the particularreceiver has locked.

If interference calls for use of an interference-adaptive receiver asdescribed above, a further improvement in system performance may beattainable if it can be determined that the receiver sections in oneregion of the building are all experiencing interference atapproximately the same one or more periods in their frequency sweep. If,in addition to the normal control signal packets and acknowledgementpackets, transceivers can be directed to transmit long term interferencepatterns to the building computer such that any patterns affectingmultiple transceivers can be identified, at some increase in operationaland communication complexity the transmitter sections in that region canbe directed not to transmit when their frequency sweep is passing thatapproximate frequency. The usefulness of this technique will bedependent partly on the stability of the transmitter frequency/timesweep relationship over a period of minutes.

In addition to the control functions described, the system is applicableto many situations where the building computer can control many deviceswhich are affected by the same environmental factor or building controldecision. For example, remotely controlled sun blinds are effective insome regions, to reduce heating or air conditioning costs. The controlunits for these blinds can easily be included in the network, at a lowercost than providing a local sensor and stand-alone control system. Thisis especially true where the operation of one system or set of devicesshould be taken into account when making control decisions for anothersystem, such as artificial lighting.

Artificial lighting has been described with respect to conventionalfluorescent tube luminaires with dimming ballasts. Of course, theinvention is not so limited. As dimming techniques may be developed forother light sources, these can equally well be controlled through anetwork according to the invention.

What is claimed is:
 1. A method of controlling at least one parameter ata plurality of device control points within a building,comprising:transmitting a first radio signal, having a first power levelsufficient to be received reliably at at least one of the device controlpoints, and insufficient to be received reliably at all device controlpoints within the building, from a master control point, the first radiosignal including a digital control signal relating to said parameter,receiving the first radio signal at one of the device control points,and transmitting a second radio signal, having a second power levelsufficient to be received reliably at at least another of the devicecontrol points, and insufficient to be received reliably at all devicecontrol points within the building, from said one of the device controlpoints, the second radio signal including said digital control signal.2. A method as claimed in claim 1, characterized in that the controlsignal relates to all the device control points in the building, furthercomprisingcontrolling at least a first device from said one of thedevice control points, in response to receipt of said first radiosignal, and controlling at least a second device from said another ofthe device control points, in response to receipt of said second radiosignal.
 3. A method as claimed in claim 1, characterized in that thedigital control signal relates to said another of the device controlpoints in the building, further comprisingcontrolling at least a firstdevice from said one of the device control points, independent of saidfirst radio signal, and controlling at least a second device from saidanother of the device control points, in response to receipt of saidsecond radio signal.
 4. A method as claimed in claim 1, characterized inthat said first and second power levels are the same, and are less than1 watt, and said first and second radio signals are transmitted withcarrier frequencies which are independent of each other and are in aband between approximately 900 and 950 MHz.
 5. A method as claimed inclaim 1, further comprising:transmitting said first and second radiosignals with carrier frequencies which are independent of each other andare in a band between approximately 900 and 950 MHz, after receiving thefirst radio signal, testing at said one of the device control point, todetermine if a further radio signal in said band is being received, andtransmitting said second radio signal only after determination that noradio signal in said band is being received.
 6. A method as claimed inclaim 1, characterized in that said first radio signal comprises apacket of digital information including said digital control signal,said packet further including a code signal indicative of a maximumnumber of times the packet should be retransmitted, andprior totransmitting said second radio signal said code signal is decrementedand then transmitted as part of said second radio signal with saiddigital control signal.
 7. A method as claimed in claim 1, characterizedin thatsaid parameter is a lighting dimming level, said one and saidanother device control points control first and second luminaires,respectively, said device control points being wall units permittinguser control of luminaires in respective rooms, and the digital controlsignal relates to said another of the device control points in thebuilding,the method further comprising the steps of controlling at leastthe first luminaire from said one of the device control points,independent of said first radio signal, and controlling at least thesecond luminaire from said another of the device control points, inresponse to receipt of said second radio signal.
 8. A system forcontrolling at least one parameter at a plurality of device controlunits within a building comprisinga master control unit for generatingcontrol signals which are selectively directed to a particular one, or agroup, or all of the individual control units,characterized in that thesystem comprises a corresponding plurality of low power radiotransceivers connected respectively to associated individual controlunits to form transceiver/control unit combinations, and a low powerradio transceiver connected to the master control unit, where each ofthe transceivers transmits sufficient power to communicate with at leastone other of the transceivers, but not all of them; the transceiverconnected to the master control unit comprises means for transmitting apacket of information including an address of one of said combinationsand a control signal, each combination comprises means, responsive toreceipt of a packet addressed to said combination, for controlling arespective device according to said control signal, and each combinationcomprises means, responsive to receipt of a packet addressed to anothercombination, for controlling said respective device independently ofsaid control signal, and for retransmitting said packet.
 9. A system asclaimed in claim 8, characterized in that each combination includes afirst microprocessor associated with the control unit for processingcontrol signals from sensors and user-operated control elements; and asecond microprocessor associated with the respective transceiver forcontrolling retransmission of a received packet, transmission of adevice control signal to the first microprocessor, and transmission ofan acknowledgement signal by the transceiver.
 10. A system as claimed inclaim 8, characterized in that it comprises a multiplicity of saidcombinations,at least a plurality of said multiplicity of combinationsare substantially identical, each packet transmitted by the transceiverconnected to the master control unit includes a code signal indicativeof a maximum number of times the packet should be retransmitted, andeach combination comprises means, responsive to receipt of a packetaddressed to at least another combination, for decrementing said codesignal and then retransmitting the packet including the decremented codesignal and said digital control signal.
 11. A system as claimed in claim8, characterized in that each combination comprises a memory for storinga respective table of addresses,each combination comprises means,responsive to receipt of a given packet addressed to at least anothercombination, for comparing the address of said given packet withaddresses in said respective table, and retransmitting the given packetonly if its address matches one of the addresses stored in saidrespective table.
 12. A system as claimed in claim 8, characterized inthat each transceiver transmits at a frequency within a given band,independent of the frequency within that band at which the most recentlyreceived packet had been transmitted,each combination comprises means,responsive to receipt of a packet which should be retransmitted, fordelaying for a randomly determined period of time, and then testing todetermine if a further radio signal in said band is being received, andtransmitting said second radio signal only after determination that noradio signal is being received in said band.
 13. A system as claimed inclaim 8, characterized in that each of the transceivers is substantiallyidentical, and transmits at a power less than 1 watt within a bandbetween 900 and 950 MHz.
 14. A system as claimed in claim 8,characterized in that each of the transceivers is substantiallyidentical, and transmits at a power less than 50 milliwatts within aband between approximately 905 and 928 MHz.
 15. A system as claimed inclaim 8, characterized in that each respective combination includes afirst microprocessor associated with a control unit for luminaireswithin an area of the building adjoining the respective combination, forprocessing light dimming control signals from sensors and user-operatedcontrol elements; and a second microprocessor associated with therespective transceiver for controlling retransmission of a receivedpacket, transmission of a device control signal to the firstmicroprocessor, and transmission of an acknowledgement signal by thetransceiver.
 16. A system as claimed in claim 8, characterized in thatsaid control signals include lighting dimming signals,the systemcomprises a multiplicity of said combinations arranged for controllingluminaires within areas of the building adjoining the respectivecombinations, at least a plurality of said multiplicity of combinationsare substantially identical, each packet transmitted by the transceiverconnected to the master control unit includes a code signal indicativeof a maximum number of times the packet should be retransmitted, andeach combination comprises means, responsive to receipt of a packetaddressed to at least another combination, for decrementing said codesignal and then retransmitting the packet including the decremented codesignal and said digital control signal.
 17. A system as claimed in claim8, characterized in that each respective combination includes amicroprocessor associated with a lighting control unit for luminaireswithin an area of the building adjoining the respective combination, forprocessing light dimming control signals from at least a user-operatedcontrol element,each transceiver transmits at a frequency within a givenband, independent of the frequency within that band at which the mostrecently received packet had been transmitted, each combinationcomprises means, responsive to receipt of a packet which should beretransmitted, for delaying for a randomly determined period of time,and then testing to determine if a further radio signal in said band isbeing received, and transmitting said second radio signal only afterdetermination that no radio signal is being received in said band.
 18. Asystem as claimed in claim 17, characterized in that each of thecombinations is substantially identical, and transmits at a power lessthan 1 watt within a band between 900 and 950 MHz.
 19. A system asclaimed in claim 17, characterized in that each of the transceivers issubstantially identical, and transmits at a power less than 50milliwatts within a band between approximately 905 and 928 Mhz.
 20. Amethod for managing a building network control system, the methodcomprising executing the following steps in a network, the networkcomprising a master node and at least first and second non-master nodeswithin a building:from the master node, wirelessly transmitting digitalcontrol information to the first non-master node at a power which issufficient to reach the first non-master node, but insufficient to reachthe second non-master node; from the first non-master node, wirelesslyrepeating the digital control information at a power sufficient to reachthe second non-master node; and receiving the digital controlinformation at the second non-master node, so that the second non-masternode can respond to the control information.
 21. The method of claim 20wherein the non-master nodes are device control points in the building.22. The method of claim 21 further comprising the step of, in responseto the digital information, in each of the first and second non-masternodes, effecting respective analogous device control actions inrespective associated first and second devices.
 23. The method of claim21 wherein the device control points are for controlling buildinglighting.
 24. A device control point for use in a building controlnetwork the device control point including:transceiver means suitablefor receiving and transmitting wireless communication of controlinformation eitherfrom and to other similar device control points;and/or from and to a master node;means, responsive to said controlinformation, for controlling an associated device when said controlinformation specifies control of the associated device; and causing thetransceiver to repeat said control information when said controlinformation specifies control of a device not associated with the devicecontrol point.